Circuit interrupter



March 1954 G. B. CUSHING CIRCUIT INTERRUPTER 2 Sheets-Sheet 1 Filed May15 1951 Fig.3.

Insulation Fig.l.

INVENTOR George B.Cushin G EJ WITNESSES:

March 23, 1954 B cusH N 2,673,272

CIRCUIT INTERRUFTER Filed May 15, 1951 2 Sheets-Sheet 2 Insulation A f AWITNESSES: INVENTOR 54 George B.Cu shing.

Patented Mar. 23, 1954 UNITED STATES PATENT OFFICE CIRCUIT INTERRUPTERGeorge B. Cushing, Turtle Creek, Pa., assignor to Westinghouse ElectricCorporation, East Pittsburgh, Pa., a corporation of PennsylvaniaApplication May 15, 1951, Serial No. 226,507

25 Claims. 1

This invention relates to circuit interrupters in general, and, moreparticularly, to arc-extinguishing structures therefor.

A general object of my invention is to provide an improved fluid blasttype of circuit interrupter which will be more easily inspected, andwhich will be more economical to manufacture and to service.

'A further object is to provide an improved circuit interrupter whichwill have reduced dimensions, and yet despite such reduced size, willhave as equally effective operating characteristics as interrupters ofthe same type heretofore manufactured.

Still a further object is to provide an improved arc-extinguishingstructure, the walls of which are of cylindrical form in which one ormore venting inserts are provided. Preferably such inserts are mountedin apertures provided in the walls of the casingso that the insert mayextend internally across the casing.

Further objects and advantages will readily become apparent upon readingthe following specification taken in conjunction with the drawings inwhich:

Figure 1 is a side elevational view, partially in vertical section, ofan improved arc-extinguishing unit embodying my invention, the contactstructure being shown in the partially open circuit position;

Fig. 2 is an end elevational view of the insert employed in theextinguishing unit of Fig. 1;

Fig. 3 is a side elevational view of the insert pictured in Fi 2;

Fig. 4 is a plan view in section, taken along the line IV-IV of Fig. 3;

Fig. 5 is a side elevational view of a .complete circuit interrupter,the tank being partly broken away to show the internal constructionthereof, and the contact structure being shown in the closed circuitposition; and

Fig. 6 is a plan view in section taken along the line VI-VI of Fig. 1.

Referring to the drawings, and .more particularly to Fig. 1 thereof, thereference numeral l designates a terminal bushing extending downwardlyfrom the cover 2 of a tank 3, with particular attention being directedto Fig. 5, the latter showing a complete circuit interrupter of the typeconsidered.

Extending interiorly through the terminal bushing I is a terminal stud 4which is exteriorly threaded. The lower end of the terminal stud 4 isthreadedly secured to, and clamped by, a contact foot :5, bolts 6 beingemployed to clamp the contact foot 5 to the lower end of the terminalstud 4.

Preferably, the contact foot 5 is an integral part of a cupshaped topdome casting I, the lower end of which is internally threaded, as at 8,to fixedly maintain in position an insulating cylindrical casing ID.

A relatively stationary pressure-generating contact H is pivotallymounted interiorly within the top dome casting I in any suitable manner,and cooperates with an intermediate contact H. to establish apressure-generating arc l3 Within a pressure-generating chamber,generally designated by the reference numeral M.

The intermediate contact I2 is spring-biased downwardly, in the openingdirection, and is moved upwardly in the closing direction against thespring pressure by a lower movable rodshaped interrupting contact I6.Preferably, the interrupting contact I6 is fixed to the end of aconducting bridging member I! (Fig. 5) which is vertically actuated inits opening and closing movements by an insulating operating or lift rodIt. The operating rod l8 may be actuated by any suitable operatingmechanism, which forms no part of my invention.

It will be observed that the diametrical opposite walls of the casingII] have mounting apertures [9 provided therein, within which is placedand. mounted a multi-orifice venting insert, generally designated by thereference numeral 20. The multi-orifice venting insert 20 is preferablymolded out of any suitable insulating molding material so that it is anintegral unit. Figs. 2-4 more clearly show the construction of theunitary multi-orifice venting insert 20. It will be noted that theventing insert 20 has two pairs of venting passages 21, which extendfrom a central passage 22 to the region exteriorly of the extinguishingunit 23, as more clearly shown in Figs. 1 and Also the venting insert 20has a pair of inlet passages 24 which communicate between the centralpassage 22 and the interior 25 of the extinguishing unit 23. .Fig. 6 maybe referred to in this connection.

Preferably, the venting insert 20 has a pair of ring-shaped end portions26 which fit within the mounting apertures 19 of the casing H). Theouter ends of the venting passages 2| extend laterally beyond thering-shaped end portions 26 to cooperate with aretainer ring 21, moreclearly shown in Figs. 1 and 6. Machine screws 28 fixedly secure theretainer ring 21 in position, and their interior ends 28a preferablymain,-

tain in position a centrally apertured lower closure plate 29, throughwhich the lower movable interrupting contact l6 moves during the openingand closing operations of the interrupter.

The operation of my improved arc-extinguishing unit will now bedescribed. During the opening operation, suitable mechanism, responsiveeither to manual operation or to the existence of overload conditionsexisting in the circuit controlled by the interrupter, causes downwardopening motion of the insulating lift rod l8 (Fig.

The downward opening motion of the lift rod [8 causes correspondingdownward opening motion of the conducting bridging member H and the twointerrupting contacts I6 disposed at the outer edges of the bridgingmember I1.

Referring to Fig. 1, the downward opening motion of the rod-shapedinterrupting contact 16 will first permit the contact pressure spring,not shown, associated with the pressure-generating contact 5 l to permitslight downward travel of the pressure-generating contact ll. Furtherdownward opening travel of the interrupting contact IE will be followedby the intermediate contact I2 due to the accelerating spring pressure,not shown, associated therewith. The consequent separation between thepressure-generating and intermediate contacts l I, [2 will cause theestablishment of a pressure-generating arc l3, as shown in Fig. 1.

Continued downward opening motion of the interrupting contact IE willresult in a separation between the intermediate and interruptingcontacts [2, Hi, the intermediate contact 12 having been halted in itsdownward opening travel by a suitably disposed stop, not shown. Thisresults in the establishment of an interrupting are 30 between theintermediate and interrupting contacts l2, H5 through the centrallydisposed arcing passage 22 of the venting insert 20, as shown moreclearly in Fig. l.

' Since liquid, such as oil 31, fills the tank 3 to the level 32 (Fig.5) oil will completely fill the extinguishing unit 23, and theestablishment of a pressure-generating arc l3 will generate pressurewithin the pressure-generating chamber 14 internally within the casingin. This pressure will be communicated downwardly through the region andwill act through the inlet passages 24 in the manner indicated by thearrows of Figs. 1, 3, 4 and 6. Also oil will be forced downwardlythrough the top orifice 33 of the insert 20, and upwardly through thebottom orifice 34 of the insert 20, as more clearly shown in Fig. 1.

The orifices 33 and 34, together with the intermediate orifices 35, 36,the latter being disposed intermediate the venting passages 2| and thecentral inlet passage 24, all serve to direct fluid, in this instanceoil, through the central arc passage 22 and into engagement with theinterrupting are 30.

Following contact with the interrupting arc .30,.such liquid passes outof casing l0 through the two pairs of exhaust passages 2|, as indi-'cated by the arrows in Figs. 1, 3, 4 and 6. Deionization and extinctionof the interrupting are soon follows, and the circuit is interrupted.The interrupting contact [6 moves downwardly with the cross-bar l1 andlift rod l8 to the dotted line open circuit position, as indicated bythe reference numeral 31 of Fig. 5, thereby introducing two seriallyrelated isolating gaps in the circuit. i

During the closing operation, it is merely nec' essary to cause upwardmovement of the operating rod [8, which raises the cross-bar l1 and theinterrupting contacts [6. The latter enter the apertures 38 provided inthe extinguishing units 23, pass along the arcing passages 22, pick upthe intermediate contacts 12 and raise the latter into contactingengagement with the pressure-generating contacts II against the springpressure associated with the intermediate contacts 12. The circuit isthen closed. Obviously, in the closed circuit position of theinterrupter the electrical circuit therethrough includes the terminalstud 4, extending through the left-hand terminal bushing I, the contactfoot 5, pressuregenerating contact ll, intermediate contact 12, lowerinterrupting contact 16, conducting bridging member I! and through theright-hand extinguishing unit 23 in similar fashion to the right-handterminal bushing 39 (Fig. 5).

Referring to Fig. 5, it will be observed that I show modifiedarc-extinguishing units 40 serially connected, and each unit 40employing a pair of venting inserts 20, the latter being angularlydisplaced from each other, as shown in Fig. 5, to thereby insure a moreuniform dissemination of the exhaust arc gases exteriorly of theextinguishing units 40. In other words, in order to prevent a possibleexternal fiashover, it is desirable to prevent the exhause arc gases,which are of a low dielectric strength, from being too concentrated atany one point. Therefore, by angularly displacing the position of theventing inserts 20, as shown in Fig. 5, the arc gases 1 are moreuniformly disseminated, and yet advantage may be takenof my improvedinsert construction for interrupting the higher voltages, where a longerlength of interrupting are 30 may have to be interrupted.

Preferably, where only two venting inserts are employed for eachextinguishing unit, the inserts are angularly disposed approximatelydegrees so that the vertical spacing between axes of the inserts can beless than the diameter of the end portions 26 without interference, andorifices can be spaced at regular intervals in the interrupting gap.Also, in addition to the retainer ring 21 for the lower venting insert20, an additional retainer ring 4| may be employed on the top side ofthe upper venting insert 20, as shown in Fig. 5.

Heretofore, the multi-orifice structure associated with the interruptingarc 30 has been formed by the contiguous superpositioning of a pluralityof fibre insulating plates, suitably configured. Reference may be had toUnited States Patent 2,467,760, issued April 19, 1949 to Leon R. Ludwig,Benjamin P. Baker and Winthrop M. Leeds, and assigned to the assignee ofthe instant application for a description of the general type of plateswhich were used.

Common practice, such as disclosed in this patent, employed fibre platesor I 6 inch thick, in quantity exceeding 30. These, together withspecial fibre plates, end plates and castings were bolted into a unitwith insulated tie rods. Although such a construction has proved to behighly effective in extinguishing the arcs, yet it is obvious that themanufacturing cost is appreciable because large areas of the originalfibre must be removed to provide oil channels, openings for tie rods,vent passages and clearance for the contacts. Moreover, a fibre Wallmust be maintained between the high pressure region within thearcextinguishing stackand theoil external to the age-ra es stack;otherwise leakage will upset the controlled oil now. However, because ofits laminar construction, no longitudinal tensile strength can bederived from this wall, and tie-rods must be provided for this function.It has generally been necessary to have an extremely large number ofcomponents which must be carefully manufactured and assembled.Replacement of the orifices can be accomplished only by completedisassembly, and the stack length is variable according to the moisturecontent of the fibre plates.

The design which I propose, and which is described above, is amulti-orifice structure em bodied in a heavy walled, high strength,insulating tube. The venting means 20, which I provide in the region ofthe interrupting gap 30, is maintained in position by a tubular retainer21, which is slid on the bottom of the interrupter and attached byscrews 28. Th cooperation between the upper edge of the retainer ring 21and the lower side 42 (Fig. l) of the lower venting passage 2l preventsshifting and rotation of the venting means 20 after installation.

This multi-orifice venting insert '20, placed in a tube in the disclosedmanner, possesses several advantages. First, the tube It) can bedesigned to contain the internal pressures, and yet can be relativelythin-walled because of its favorable circular shape. So designed, italso will withstand tensile stresses along its axis, eliminating anyneed for tie-rods, which were used in the construction set out in Patent2,467,760. Additional space will accordingly be available for internaloil flow.

The venting means 20, which I have illustrated, makes possible theincorporation of multi-flow action in a tubular structure with severaladvantages. It is extremely simple, being a single molded insertinstalled in holes drilled on the diameter of tube I yet this simplicityincorporates all of the inlet channels and vent passages, commonlysupplied in the interrupting gap of a multi-flow type of interrupter, asset out in the aforesaid patent.

There is little opportunity for leakage, the intersection of the tube [0and venting means 20 being a simple curve which may, if desired, besealed with an O ring type gasket. The tubular construction of the wallsencasing th venting passages 2| in the venting insert 20 providesconsiderable resistance to pressures within chamber 25.

Internal forces within chamber 25 tending to expel the venting insert 20from the arc-extinguishing unit 23 are balanced, giving zero nethorizonta1 force. Replacement of the venting means 20 entails simplyremoval of retainer ring 21, attached with screws 28 and a complete newset of interrupting orifices 33-36, provided by a new insert 20, willreturn a heavily used extinguishing unit 23 to good as new condition.Functionally, space not required for venting is available as largecross-sectional oil flow channels; and the elimination of the tie-rows,previously'used, will permit the entire arc-extinguishing unit to havesmaller external dimensions.

Where higher voltage applications require additional multi-floworifices, the venting means 20 can be installed in multiple, asindicated in Fig. 5, successive units 20 being oriented on otherdiameters around tube II). By this means, the vented gas can bedistributed around the interrupter to reduce the hazard of externalflashover. Also spacing between adjacent venting means 7.0 can besmaller than the diameter of the ends 6 of these components. Thedistribution of vented gas would be particularly desirable in a low oilcontent type of breaker where the gases are restricted by the closelyencasing tubing.

The venting means 20 could also be applied to an impulse type ofinterrupter, where a piston substitutes for the pressure-generating arcl3 in raising the internal pressure of the interrupter and therebycausing oil flow. Thus certain features described in my invention makeit particularly well adapted to low oil content breakers of either theself-generated gas blast or impulse type.

The foregoing description of two embodiments of my invention show how Ihave constructed a multi-fiow type of interrupter in a tube with meansof venting the tubular constructed interrupter to give multi-fiowcharacteristics. Also my improved interrupter gives improvedinterrupting performance by providing larger section oil channels withno increase in over-all size. The foregoing construction permits easyreplacement of interrupting orifices, and the inserts can readily beformed of a suitable molding material.

Although I have shown and described specific structures, it is to beclearly understood that the same were merely for the purpose ofillustration, and that changes and modifications may readily be madetherein by those skilled in the art, without departing from the spiritand scope of the appended claims.

I claim as my invention:

1. A circuit interrupter including a casing, means establishing an arcwithin the casing, interrupting passage structure for extinguishing theare, a venting insert having an exhaust passage constituting at least aportion of said interrupting passage structure and exhausting fluid fromthe arc away from th interrupting passage structure and out of thecasing, the side Wall of the casing having an aperture to assist insupporting the insert, and the insert being laterally removable from theside wall of the casing.

2. A circuit interrupter including a casing, a pair of mountingapertures provided diametrically in the side walls of the casing, meansfor establishing an arc within the casing, interrupting passagestructure for extinguishing the arc, a venting insert having exhaustpassages therein constituting at least a portion of said interruptingpassage structure and exhausting fiuid from the arc and out of thecasing, the venting insert being mounted in the apertures and extendingdiametrically across the casing, and the venting insert being laterallyremovable from the side wall of the casing.

3. A circuit interrupter including a casing, means establishing an arewithin the casing, interrupting passage structure for extinguishing thearc, a plurality of venting inserts having exhaust passages andconstituting at least a portion of said interrupting passage structure,each venting insert exhausting fluid from the arc out of the casing, apair of apertures provided in the side wall of the casing angularlydisplaced to mount the venting inserts, and each venting insert beinglaterally removable from the side Wall.

of the casing.

4. A circuit interrupter of the liquid break type including a tubularcasing, means for establishing a pressure-generating arc and a seriallyrelated interrupting are within the casing, the side walls of the casingadjacent the interrupting are having apertures provided therein, and aventing insert laterally removable from the side wall.

of the casing extending between the apertures internally across thecasing.

. '5. A circuit interrupter of the liquid break type including a tubularcasing, means for establishing a pressure-generating arc and a seriallyrelated interrupting are within the casing, the side walls of the casingadjacent the interrupting are having apertures provided therein, aplurality of venting inserts laterally removable from the side wall ofthe casing extending between the apertures internally across the casing,and each insert being angularly displaced with respect to the adjacentlydisposed insert to disseminate the ex haust arc gases.

6. A circuit interrupter including a casing, means establishing an arewithin the casing, a multi-orifice insert having one or more exhaustpassages therein for exhausting fluid away from the arc and out of thecasing, the side wall of the casing having an aperture to assist insupporting the multi-orifice insert, and the multi-orifice insert beinglaterally removable from the side wall of the casing.

7. A circuit interrupter including a pressure chamber, a longitudinalcasing defining at least a part of said pressure chamber, means forestab lishing an are within the longitudinal casing, a pair of mountingapertures provided diametrically in the side walls of the casing, amulti-crifice insert having one or more exhaust passages therein forexhausting fluid away from the are and out of the casing, themulti-orifice insert extending interiorly across the casing between saidapertures, and the multi-orifice insert being laterally removable fromthe side wall of the casing.

8. A circuit interrupter including a cylindrical casing, meansestablishing an are within the casing generating pressure therein, aplurality of multi-orifice inserts laterally removable from the sidewall of the casing, the side walls of the casing having aperturesprovided therein so that the multi-orifice inserts may extend internallyacross the casing, and each multi-orifice insert being angularlydisplaced with respect to the adjacently disposed multi-orifice insertto disseminate the exhaust arc gases.

9. A circuit interrupter of the liquid break type including a. tubularcasing, means for establishing a pressure-generating arc and a seriallyrelated interrupting are within the casing, the side walls of theadjacent the interrupting are having apertures provided therein, and amultiorifice insert extending between the apertures internally acrossthe casing and laterally removable from the side wall of the casing.

10. A circuit interrupter of the liquid break type including a tubularcasing, means for establishing a pressure-generating are and a seriallyrelated interrupting arc within the casing, the walls of the casing adjacent the interrupting are having apertures provided therein, aplurality of laterally removable multi-orifice inserts extending betweenthe apertures internally across the casing, and each multi-orificeinsert being angularly displaced with respect to the adjacently disposedmulti-orifice insert to disseminate the exhaust arc gases.

11. A circuit interrupter including a cylindrical casing, meansincluding a movable contact for establishing an are within the casinggenerating pressure therein, a removable venting insert having at leasta pair of opposed exhaust passages and one or more inlet passagesprovided therein, said insert having a centrally-disposed arc passage toaccommodate the movable contact, and the walls of the casing havingapertures provided therein so that the insert may extend internallyacross the casing with the opposed exhaust passages exhausting throughthe apertures.

12. A circuit interrupter including a pressure chamber, a longitudinalcasing defining at least part of said pressure chamber, aperturesprovided diametrically in the walls of the casing, a removable ventinginsert extending internally or the casing between the apertures, andsaid venting insert having at least a pair of opposed exhaust passagesprovided therein to exhaust in substantially opposite directions out ofthe apertures.

13. A circuit interrupter including a cylindrical casing, meansestablishing an are within the casing generating pressure therein, aplurality of each insert having at least a pair of opposed exhaustpassages provided therein, the exhaust passages exhausting insubstantially opposite directions out through the apertures, andeachinsert being angularly displaced with respect to the adjacentlydisposed removable insert to disseminate the exhaust arc gases.

14. ,A circuit interrupter of the liquid break type including a tubularcasing, means for establishing a pressure-generating arc and a seriallyrelated interrupting arc within the casing, the walls of the casingadjacent the interrupting are having apertures provided therein, aremovable venting insert having at least a pair of exhaust passagesprovided therein extending between the apertures internally across thecasing, and the exhaust passages exhausting in substantially oppositedirections out through the apertures.

15. A circuit interrupter including a cylindrical casing, meansincluding a rod-shaped movable contact for establishing an arc withinthe casing generating pressure therein, a removable vent-' ing inserthaving a centrally disposed passage provided therein for accommodatingmovement of the rod-shaped contact, said venting insert also having oneor more exhaust passages and at least a pair of inlet passages, thewalls of the casing having apertures provided therein so that theventing insert may extend internally across the casing, and the exhaustpassages exhausting in substantially opposite directions out through theapertures.

16. A circuit interrupter including a cylindrical effect the lattersextinction, a removable vent-. ing insert having a centrally disposedpassage.

for accommodating motion of the movable interrupting contact, saidventing insert having at least a pair of opposed exhaust passages and atleast a pair of cooperating inlet passages, and the walls of the casinghaving apertures provided therein so that the venting insert may extendinternally across the casing.

17. A circuit interrupter of the liquid breal i' type including anarc-extinguishing imit immersed in liquid, the unit including acylindrical casing, contact means including a rod-shaped. movableinterrupting contact for establishing a pressure-generating arc and aserially related interrupting are within the casing, a removable ventinginsert having a centrally disposed passage therein to: accommodatemotion of the rod" shaped movable interrupting contact, said ventinginsert also having at least a pair of opposed exhaust passages and atleast a pair of cooperating inlet passages, and the walls of the casinghaving apertures provided therein so that the insert may extendinternally across the casing.

18. A circuit interrupter including a casing, means establishing an arcwith-in the casing, a multi-orifice insert having one or more exhaustpassages therein for exhausting fluid away from the arc and out of thecasing, the side wall of the casing having an aperture engaged by themulti-orifice insert to assist in supporting the multi-orifice insert,the multi-orifice insert being laterally removable as a unit from theside wall of the casing, and a space on each side of the multi-orificeinsert between the insert and the inside wall of the casing.

19. A circuit interrupter including a casing, means including arod-shaped contact movable longitudinally of the casing for establishingan are within the casing, a multi-orifice insert having one or moreexhaust passages therein for exhausting fluid away from the are and outof the casing, said multi-orifice insert having an arc passage extendinglongitudinally of the casing and in which the movable rod-shaped contactmoves, the side wall of the casing having an aperture to assist insupporting the multi-orii'lce insert, the multi-orifice insert beinglaterally removable from the side wall of the casing, a space on eachside of the multi-orifice insert between the insert and the inside wallof the casing, and one or more inlet passages leading from the space inthe casing to the arc passage.

20. A circuit interrupter including a cylindrical casing, meansestablishing an are within the cylindrical casing, a multi-orificeinsert having one or more exhaust passages therein extending out of thecasing for exhausting fluid away from the arc and out of the casing, theside wall of the cylindrical casing having an aperture to assist insupporting the multi-orifice insert, and the multi-orifice insertincluding its one or more exhaust passages having transverse dimensionsno greater than the dimensions of the aperture and being laterallyremovable from the side wall of the casing through the aperture.

21. A circuit interrupter including a cylindrical casing of insulatingmaterial, means establishing an arc within the cylindrical casinglongitudinally thereof, a multi-orifice insert of insulating materialhaving one or more exhaust passages therein for exhausting fluid awayfrom the arc and laterally out of the cylindrical casing, the side wallof the cylindrical casing having an aperture to assist in supporting themulti-orifice insert and through which the fluid exhausts, themulti-orifice insert being laterally removable from the side wall of thecylindrical casing, and a space having the shape of a segment of acircle on each side of the multi-orifice insert between the insert andthe inside wall of the cylindrical casing.

22. A circuit interrupter including a casing, means including a movablerod-shaped contact for establishing an are within the casing, amultiorifice insert having one or more exhaust passages therein forexhausting fluid away from the arc and out of the casing, saidmulti-orifice insert having a central arc passage in which the movablerod-shaped contact moves, the side wall of the casing having an apertureinto which one end of the multi-oriiice insert extends to assist insupporting the multi-orifice insert, said rodshaped contact beingmovable to a position entirely out of the multi-orifice insert, and themulti-orifice insert then being laterally removable from the side wallof the casing through said aperture.

23. A circuit interrupter including a pressure chamber, a longitudinalcasing defining at least part of said pressure chamber, aperturesprovided diametrically in the walls of the casing, a removable ventinginsert extending internally of the casing between the apertures, saidventing insert having at least a pair of opposed exhaust passagesprovided therein to exhaust in substantially opposite directions out ofthe apertures, the whole venting insert which extends between the twoapertures being laterally removable from the side wall of the casingthrough one of the apertures, and a retaining means disposed externallyof the casin and engaging a portion of the venting insert to assist inretaining the same in a fixed position in the casing.

24:. A circuit interrupter including a, casing, means establishing anare within the casing, a mu1tiorifice insert having one or more exhaustpassages therein for exhausting iiuid away from the arc and out of thecasing, the side wall of the casing having an aperture to assist insupporting the multi-orifice insert, the multi-orifice insert beinglaterally removable from the side wall of the casing through theaperture, and a retaining member disposed externally of the casing andengaging a portion of the multi-orifice insert to assist in retainingthe same in a fixed position in the casing.

25. A circuit interrupter including a casing, means including a movablerod-shaped contact for establishing an are within the casing, amultiorifice insert having one or more exhaust passages therein forexhausting fluid away from the arc and out of the casing, saidmulti-orifice insert having a central arc passage through which themovable rod-shaped contact moves, the side wall of the casing having anaperture to assist in supporting the multi-orifice insert, themultiorifice insert being laterally removable from the side wall of thecasing, and at least one end of the central arc passage being relativelyopen during the are drawing process to permit fluid in the casingadjacent the multi-orifice insert to flow into the arc passage.

GEORGE B. (BUSHING.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,470,611 Friedrich May 17, 1949 2,477,837 Strom Aug. 2, 1942FOREIGN PATENTS Number Country Date 450,405 Great Britain July 16, 1936

